Biofuels

Most of us encounter biofuel every time we go to the gas station, where as much as 10% of what goes into the tank is ethanol (grain alcohol)—a renewable fuel source that burns cleaner than gasoline. In the United States, ethanol is made chiefly from corn using basically the same methods that produce whiskey or other distilled spirits: Corn is mashed, yeast is added, and fermentation converts the sugars into alcohol, which is distilled out and mixed with gasoline.

Corn (or sugar cane in Brazil) is the most readily used feedstock because its starches (or sugar in the case of sugar cane) are easily accessible. But ethanol can also be made from many kinds of comparatively inexpensive or waste biomass, including field-crop stalks and leaves (stover) typically left in the field after harvesting, along with straw, wood chips, and sawdust. (Note, however, that low-cost biomass does not necessarily mean low-cost energy. The technology to make that possible has not been adequately developed.) 

No matter how it is produced, ethanol adds oxygen content to gasoline, increases its octane rating, and generally lowers the amount of air pollution produced by vehicles.

All these sources have high concentrations of cellulose, a tough chain of complex carbohydrates that is indigestible by humans and serves as the principal source of our dietary fiber. Termites and ruminants such as cattle, however, are hosts to microbes with the necessary enzymes to break down cellulose. The chemistry involved is well understood and artificial processing systems have been built to achieve similar results.

According to the U.S. Department of Energy, 1 ton of corn grain can yield as much as 124 gallons of ethanol; but 1 ton of corn stover can yield nearly as much—113 gallons. Exploiting such sources, however, requires extra pre-treatment and chemical steps to break down the cellulose into simpler forms (or through other technologies such as gasification), and cellulosic ethanol production to date has been limited while research continues.

No matter how it is produced, ethanol adds oxygen content to gasoline, increases its octane rating,* and generally lowers the amount of air pollution produced by vehicles. Prior to 2006, the main oxygenator in gasoline was MTBE (methyl tertiary-butyl ether), which has been largely phased out because of its potential effect on water supplies. Adding ethanol to gasoline, however, reduces the energy content per gallon.

Although a great deal of energy is expended in making ethanol, including farming, harvesting, and transportation of the feedstock as well as the energy used in its manufacture, the resulting alcohol still contains more energy than the processes used to produce it—a positive energy balance—but not necessarily a positive carbon dioxide (CO2) balance. However, ethanol production facilities are a particularly interesting opportunity for carbon capture and storage because they produce a nearly pure stream of CO2.

Fuel for diesel-powered vehicles can also be made from biomass sources by exploiting the oil component of various plants, animal fats, algae, and even recycled restaurant greases. Biodiesel, which has a somewhat lower energy content than petroleum-based fuel, is not a new idea. At the Paris Exhibition of 1900, an engine design by Rudolf Diesel ran entirely on peanut oil. 

Modern biodiesel is either blended with petroleum diesel fuel in various proportions (a 20% blend called B20 is becoming common) or used at 100% (B100). B100 does not perform well at low temperatures and its use is incompatible with many diesel engine designs. In the near term, blends will make the largest renewable contribution to America’s diesel fuel supply.

The size of that contribution, however, will depend to a great extent on the supply and price of competing fuels. Only in an economic environment characterized by high oil prices, technological breakthroughs, and a high implicit or actual carbon price would biofuels be cost-competitive with petroleum-based fuels.

* Octane rating is a measure of how highly a fuel–air mixture can be compressed before it ignites.